The invention relates to voltage regulators, particularly those intended for use in automotive applications. Under certain conditions, an automotive voltage regulator will be required to provide a plurality of outputs so that several independent devices can be supplied with a regulated voltage. These outputs each employ an isolator stage that permits independent or isolated load supplies. One of the main characteristics of an automotive system is the propensity of various parts of the chassis ground to assume different potentials. This is a well known fact of life in the automotive world. The various chassis grounds can develop as much as a +4-volt differential. Thus, when an output becomes shorted to ground, it can be as low as -4 volts. Another problem can develop where the regulator output becomes shorted to a higher than normal positive potential. For example, as much as 26 volts can inadvertently become associated with the output terminal. As a result, the automotive voltage regulator outputs under adverse conditions can be subjected to voltages that may vary from +26 volts to -4 volts. It is desired that the regulator survive such extremes without damage and that for plural outputs the fault conditions applied to one output will not adversely affect the other outputs.
Another voltage regulator characteristic involves its output impedance. If the circuit pass transistor is of PNP polarity, as is often the case, the collector is connected to the output terminal. This is the high impedance transistor element and this connection produces an instability that requires a relatively large bypass capacitor as a cure. This condition is presented in detail in a U.S. Pat. No. 4,928,056, by Robert A. Pease. This patent is titled A STABILIZED LOW-DROPOUT VOLTAGE REGULATOR CIRCUIT, issued May 22, 1990, and is assigned to the assignee of the present invention. The teaching in this patent is incorporated herein by reference. Typically, the use of a PNP output pass transistor will require a minimum of ten microfarads bypass capacitance. Preferably a tantalum capacitor is employed. In the present invention, an NPN output pass transistor is employed which requires that the low impedance emitter terminal be connected to the output terminal. This configuration permits the use of a relatively small 0.06 microfarad capacitor. While the use of a small capacitor is not of much economic significance in a single voltage regulator, a plural output device can require the use of several relatively costly capacitors. This can be significant.
As a further consideration, when the voltage regulator is fabricated in the form of a monolithic integrated circuit (IC), the chip area is substantially taken up by the output pass transistor. When using an NPN type of output pass transistor, we have found that much less chip area is required as opposed to using a PNP type. Therefore, the invention also produces an IC area economy.